Drilling and completing oil and gas wells is a highly expensive undertaking since oil and gas bearing formations are generally located many thousand of feet below the surface of the earth. As is known to those of ordinary skill in the art, deviated wells are commonly utilized to improve production, reduce costs, and minimize environmental impacts. Wellbores including vertical, doglegged, and horizontal sections are now common. For example, extended reach wellbores commonly extend vertically only a few thousand feet downward from the surface but may extend many thousand feet (even tens of thousands of feet) horizontally.
Completing oil and/or gas wells requires deploying a completion assembly (also referred to herein as a completion string), for example, including a casing string or a sand screen in a previously drilled borehole. The completion assembly may also include a production combination string that can include many different types of downhole production or well stimulation devices (e.g. inflatable packers) and can be deployed in either a cased or open wellbore. In many completion applications, a casing string is lowered into the borehole under the influence of the Earth's gravitational field. In highly deviated, horizontal, and/or extended reach wellbores, deployment of the casing string can be problematic. For example, when the wellbore is highly deviated and of substantial length, the longitudinal frictional forces (referred to herein as drag) along the length of the casing become so great that the casing can become damaged or even stuck in the well.
One method that is sometimes used to deploy a casing string in a wellbore is to rotate the assembly during deployment. While rotation of the casing string tends to reduce drag, it also subjects the string to high torsional stresses. Conventional casing tends to be highly susceptible to both axial and torsional stresses. These axial and torsional stresses are known to buckle or otherwise damage completion assembly components during installation. As a result, high strength casing components (referred to in the industry as “premium joints”) are required when using rotation. This adds significant expense to a conventional casing operation and is therefore undesirable for many operations. Moreover, a completion assembly commonly includes one or more tubulars having slots, screens, or other openings (for example, heavy oil applications commonly employ a string of slotted casing). These openings tend to further reduce the strength of the casing and therefore further limit the axial and/or torsional load that can be applied to the string.
One disclosed method for extended reach wells is to float the casing off the bottom of the well with a dense fluid such as drilling fluid (mud). In such operations, the casing is run into the well empty with a shoe or plug deployed on the lower end. As it moves into the mud-filled well, a buoyancy force tends to float the casing string off the bottom of the well. While the buoyancy of the casing tends to reduce drag, it can also present problems. For example, floated casing has a tendency to “kick back” (up and out) of the wellbore. This kick back can be a significant safety concern and requires that the casing be firmly held at all times while it is lowered into the wellbore.
The aforementioned drag is often significant even when the casing is floated. Those of ordinary skill in the art will appreciate that a horizontal section of a wellbore is seldom perfectly straight and often includes various peaks, valleys, twists, and turns (especially in geosteering and well twinning applications). These borehole features can significantly increase friction. Moreover, a casing string including various openings (e.g., slots) is not readily floated since the drilling mud can quickly fill the casing as it is lowered into the wellbore.
Therefore, there remains a need in the oilfield services industry for improved methods for deploying a completion string in a deviated borehole. In particular, there remains a need for deployment methods that reduce drag between the casing string and the borehole wall.